1. Field of the Invention
The present invention relates to a control method and an apparatus for precisely driving a motor of a head drum assembly by using impulse generation magnets in a video cassette recorder.
2. Description of the Prior Art
Generally, a video cassette recorder is an apparatus for recording video/audio signals onto a magnetic tape and for reproducing the recorded signals. The video cassette recorder includes a main base. A driving system is mounted on the main base and draws the tape from a cartridge to run the tape around the outer surface of the head drum according to a predetermined route. A head drum assembly is utilized for recording the video signals onto the tape and for reproducing the recorded video signals.
FIG. 1 is a lateral cross-sectional view for showing a structure of a conventional head drum assembly for use in a video cassette recorder. As shown in FIG. 1, the head drum assembly comprises a rotating shaft 10 divided into an upper part 101, and a lower part 102, a rotary drum 20, a plurality of heads 21, a flange 30 with a bottom surface 31, a stationary drum 40 having a top surface 41, a pre-pressing boss 50 having a bottom surface 51, a rotor 60 having a top and lower bottom surfaces 61 and 62, a stator transformer 70, a rotor transformer 80, a stator 90, and an upper and a lower set of bearings A and B.
At the lower surface of stationary drum 40, a motor 110 including rotor 60 and stator 90 for generating the rotational force is installed. Rotating shaft 10 transmits the rotational force of motor 110 to rotary drum 20, and is rotatably inserted into rotary drum 20 and stationary drum 40. Both stator transformer 70 and rotor transformer 80 are installed between rotary drum 20 and stationary drum 40. Rotor transformer 80 is connected to rotary drum 20, and stator transformer 70 is connected to stationary drum 40.
In a reproducing mode of the VCR, the video signals reproduced by heads 21 are transmitted to an external device, e.g., an amplifier, via stator transformer 70 from rotor transformer 80 by an induction between rotor transformer 80 and stator transformer 70. On the other hand, in a recording mode of the VCR, the video signals reproduced by heads 21 are transmitted to heads 21, e.g., an amplifier, via rotor transformer 80 from stator transformer 70 by an induction between rotor transformer 80 and stator transformer 70. Accordingly, heads 21 which are mounted on the head drum assembly records or reproduces the video signals onto/from the magnetic tape.
U.S. Pat. No. 5,677,816 granted to Oh discloses a head drum assembly which is capable of preventing a resonance caused by an electrical or mechanical disturbance having a frequency which is similar to a natural vibration frequency of the head drum assembly.
FIG. 2 is a block diagram for showing a circuit configuration of a driving apparatus of the head drum assembly shown in FIG. 1. As shown in FIG. 2, the driving apparatus of the head drum assembly includes a plurality of hall sensors 200 and 210 which detect a relative rotational position of rotor 60 in accordance with a predetermined interval of a rotational angle.
A plurality of amplifiers 220 and 230 which are respectively connected to output terminals of the plurality of hall sensors 200 and 210, amplify a plurality of rotor position signals 201 and 211 detected by hall sensors 200 and 210 to a plurality of predetermined signal levels, and provides a plurality of amplified rotor position detection signals 221 and 231. Amplifiers 220 and 230 are respectively connected to a plurality of driving transistors 260 and 270 via a plurality of resistors 240 and 250. Driving transistors 260 and 270 respectively provide a plurality of exciting currents 261 and 271 to a plurality of magnetic coils 280 and 290 of stator 90 in response to amplified rotor position detection signal 221 and 231.
A plurality of frequency generator (hereinafter, referred to as "FG") magnets (not shown) for detecting a rotational position of rotor 60 are securely attached to an inner peripheral surface of a frame of rotor 60, and a plurality of hall sensors 200 and 210 which generate detection voltages in response to magnet forces of the FG magnets, are disposed in an opposite position to the FG magnets.
When the exciting currents are sequentially supplied to magnetic coils 280 and 290 of stator 90, rotor 60 continually rotates. Then, the FG magnets of rotor 60 come to rotate and provide to a control section (not shown) rotor position signals 201 and 211 which are detected by hall sensors 200 and 210. The control section controls the rotational speed of the motor of the head drum assembly based on detected rotor position signals 201 and 211.
In the above-described conventional video tape recorder, in the recording of the audio/video signals on the magnetic tape, the motor of the head drum assembly has to rotate in a stable state so as not to change a timing in a graph which depicts a signal magnitude vs. a time of the signal to be recorded. Thus, in the case where the signal is reproduced from the magnetic tape after it is recorded on the magnetic tape, in order to continually generate an ideal track timing of the magnetic tape, both the rotational speed of the head drum assembly and a running speed of the magnetic tape have to be maintained the same as in the recording.
Furthermore, since the hall sensors for detecting the rotational speed of the rotor have a relatively high price, the unit price increases in the manufacturing of the driving apparatus of the head drum assembly. Also, a reserved room is demanded in order for the hall sensors to be installed on a printed-circuit board which is secured onto a stator side of the head drum assembly.